This is why quick operating pumps have generally sucking devices--notably valves--controlled by the movement of the pump. In such equipments, the position of each suction device--particularly of each valve--depends only on the position of the corresponding piston and of the phase in which it is. Such devices present the serious disadvantage of causing, cylinder by cylinder, the opening of the suction device at the beginning of the suction stroke of the corresponding piston, viz. at a moment where the dead chamber is still at a pressure which is very close to the delivery pressure of the pump. Said dead chamber having usually a rather important volume and the compressibility of the conveyed fluids being far from negligible, the energy contained in the dead chamber is important and increases with the operating pressure of the pump. The communication established between the dead chamber and the suction causes therefore an abrupt decompression of said dead chamber, this representing not only a loss of energy but increasing also substantially the operating noise of the pump.
Thus, for example, if one considers a hydraulic pump operating at 400 bars with a hydraulic liquid such as is defined in French Standard AIR 3520, one may consider that at the usual operating temperature of 50.degree. C., the compressibility coefficient of the fluid will be of 1/15,000. A theoretical calculation shows that if the dead chamber has a volume of 25 cm.sup.3 and the cylinder content is of 10 cm.sup.3, a rotation of the skew plate over about 36.degree. will be needed (over a stroke of 180.degree. for establishing the pressure) for ensuring the rising of the pressure to 400 bars; and after the end of the delivery stroke, a rotation of about 30.degree. will be needed. As the compressibility coefficient may vary from 1/22,000 at 50.degree. C. to 1/9,500 at 150.degree. C., and as microscopic air bubbles may be present in the delivered liquid, it may happen in practice that a rotation of the pump shaft over 50.degree. is necessary for decompressing the piston chamber.
Attempts have already been made to remedy these disadvantages by foreseeing means for communicating the suction with the dead chamber, comprising a delay providing a relative decompression of the dead chamber prior to its being set in communication with the suction.
This is the case particularly with skew plate pumps which do not comprise valves but a lunule engraved on the plate, the communication being established through the piston. It is then possible to place the beginning of the lunule so that the communication is established only after of a time of decompression. But for the high pressures and the low flow rates--conditions existing for example with variable cylinder pumps--this would lead to reducing the lunule excessively, the rotation needed for the decompression being in the neighborhood of 180.degree.; moreover, for the high flow rates obtained, for reasons of compactness, through a large stroke of the piston, the passage cross-section in each piston is generally not sufficient, and this solution becomes therefore impracticable.